As many as eight US soldiers were killed in a helicopter crash, around 250 km away from Kabul, on Sunday.
The incident occurred shortly after the Afghan authorities reported the killings of seven Afghans in fresh spate of unrest near Kabul.
Officials said that the Chinook transport chopper came down in the southeastern province of Zabul, about 250 km southwest of Kabul.
"Eight coalition personnel were killed and 14 others were wounded early on Sunday when a coalition CH-47 helicopter had a sudden, unexplained loss of power and control and crashed in eastern Afghanistan," the Daily Times quoted as saying a statement issued by the US-led coalition in Afghanistan.
Investigation would verify the cause of the crash, said the statement, even as earlier it had said that the chopper came down after reporting engine failure.
Meanwhile, the NATO-led force said that British forces attacked and cleared a "major Taliban extremist headquarters" in the Helmand province in an overnight operation. It did not say how many rebels were killed.
On the other hand, the authorities reported the killing of seven Afghans in new unrest.
In Paktia province about 25 gunmen who had taken refuge in a village late on Saturday dragged five worshippers from a mosque and shot them, killing two, provincial police chief Abdul Rahman Sarjang said.
The British-led operation in Helmand overnight destroyed three major compounds and a tunnel complex that linked them, an ISAF statement said without estimating Taliban casualties.